The present invention relates to a control process for a matrix display means displaying grey levels. It more particularly applies to the field of optoelectronics and mainly to the control of liquid crystal cells, used as converters of electrical information into optical information for the real time processing of optical images and for analog display purposes.
FIG. 1a diagrammatically shows a cross bar matrix display means according to the prior art, while FIG. 1b is an exploded view thereof.
FIG. 1a shows two facing insulating walls 1, 3, which are kept spaced from one another by a seal 2 located on their peripheries and between which is introduced a material 4 having an optical feature such as an opacity, transparency, absorption, diffusion or birefringence.
Over the inner face of wall 1 is distributed a first group of m parallel electrodes M.sub.i, with i being an integer such that 1.ltoreq.i.ltoreq.m, constituted by continuous conductive strips, while over the inner face of the other wall 3 is distributed a second group of p parallel electrode columns N.sub.j, with j being an integer such that 1.ltoreq.j.ltoreq.p and also constituted by conductive strips, the m rows and p columns of electrodes crossing one another. These m rows and p columns of electrodes respectively serve to carry row signals and column signals appropriate for the excitation of material 4. This excitation can be electrical, magnetic or thermic and is dependent on the material 4 inserted between the electrode rows and columns. The excitation signals are produced in a continuous manner by a power supply source 6.
FIG. 1b, which is an exploded view of the display means, shows the first group of m electrode rows M.sub.i and the second group of p electrode columns N.sub.j distributed respectively on walls 1 and 3. The m rows and p columns of electrodes cross, so that a zone M.sub.i N.sub.j of material is defined by the overlap region of row M.sub.i and column N.sub.j. Each zone M.sub.i N.sub.j defines an elementary image point of the display means, the latter thus having mxp image points distributed in matrix-like manner.
In a particular case of the liquid crystal, for example, excitation is of an electrical type. Thus, the application of voltages to rows M.sub.i and columns N.sub.j of the electrodes produces an electric field within the liquid crystal in the corresponding overlap zones M.sub.i N.sub.j leading to a collective orientation of the liquid crystal molecules in said zones.
By utilizing the selective orientation of the molecules and the point-by-point excitation of the liquid crystal, an image appears on the complete display means while defining same in point-by-point manner.
The timing diagram of FIG. 2 makes it possible to understand the known control principle of a matrix display means making it possible to display grey levels. Signal A represents the voltage applied to a row electrode M.sub.i. Signals B, C and D represent the different voltages applied to a column electrode N.sub.j as a function of the desired display.
Signal A, B, C and D are square-wave signals of alternately positive and negative amplitude and consequently of zero mean value. The polarity inversion of these signals makes it possible to protect the display material, particularly the liquid crystal from direct currents and, consequently, to extend its life. Signal A is a pulse signal of cycle T, T being generally approximately 40 ms. Signal A is non-zero solely during a row time T.sub.L and its polarity is reversed at half of time T.sub.L. Time T.sub.L is equal to the cycle T divided by the number of rows m of the display means: T.sub.L =T/m.
Each of the rows of the display means thus receives a row signal identical to that shown in FIG. 2 at A, but delayed by a time t.times.T.sub.L with respect thereto, t being an integer such that 1.ltoreq.t.ltoreq.m-1.
The column signals B, C and D are continuous and are amplitude signals which are successively positive, then negative and having an amplitude below that of the row signals. The amplitude of these signals has a rising front 11 and a falling front 13 for each row time T.sub.L and 2m fronts on the complete time T, m corresponding to the number of rows. The column signal B is in phase opposition with the row signal A during the first time T.sub.L of cycle T.
The resulting signal, during this first time T.sub.L in a zone M.sub.i N.sub.j corresponding to the overlap of a row M.sub.i to which is applied the row signal A and a column N.sub.j to which is applied the column signal B, is a signal whose amplitude is the sum of the amplitudes in absolute values of the row signal A and the column signal B. The resulting amplitude exceeds that of the threshold voltage V.sub.s of the liquid crystal, V.sub.s corresponding to the minimum voltage required for exciting the liquid crystal. Thus, in this overlap zone M.sub.i N.sub.j, the appearance of an electric field will bring about an orientation of the liquid crystal molecules in said zone M.sub.i N.sub.j and the display of a white point.
Over the remainder of the time T, as the row signal A is zero, the resulting signal will be that of the column signal B. The amplitude of a column signal is below the amplitude of the threshold voltage, so that the resulting signal is inadequate for exciting the material. Moreover, the resulting display is that obtained during the first row time T.sub.L, i.e. white, the signal being stored for the remainder of the time T. Column signal C is identical to signal B, but is of reverse polarity. It is therefore in phase with the row signal A during the first time T.sub.L.
The resulting signal during this time T.sub.L in a zone M.sub.i N.sub.j corresponding to the overlap of a row M.sub.i, to which is applied a row signal a, and a column N.sub.j to which is applied a column signal C, is a signal, whose amplitude is the difference between the amplitudes, in absolute values, of row signal A and column signal C. The resulting amplitude is consequently below that of the threshold voltage V.sub.s. Thus, in the overlap zone M.sub.i N.sub.j, the liquid crystal is not excited and a black point is displayed.
As hereinbefore, over the remainder of time T, the resulting signal is that of the column signal C, so that the resultant display remains black.
The column signal D is identical to signal C, but is delayed by a time .delta. with respect thereto with .delta.&lt;T.sub.L.
Signal D is intermediate between signal B and signal C. For a row time T.sub.L, it is twice in succession in phase and then in phase opposition with the row signal A. At the intersection point M.sub.i N.sub.j of a row M.sub.i, to which is applied the row signal a, and a column N.sub.j, to which is applied a column signal D, there is consequently a grey display.
The phase difference .DELTA..phi. between the row signal applied to a row M.sub.i and a column signal B, C or D applied to a column N.sub.j determines the display state of the image point M.sub.i N.sub.j : EQU .DELTA..phi.=2.pi.(.delta./T.sub.L)
Thus, when .DELTA..phi.=0, point M.sub.i N.sub.j is black, when .DELTA..phi.=180.degree. point M.sub.i N.sub.j is white and finally when 0&lt;.DELTA..phi.&lt;180.degree. point M.sub.i N.sub.j is grey.
By varying .DELTA..phi., i.e. by varying the time lag .delta. of the column signal, it is possible to obtain different grey levels. The 2m fronts of the different column signals B, C and D at each cycle T lead to coupling phenomena between the rows and columns, causing a deterioration of the displayed image.
In order to reduce the coupling phenomena, in known manner the number of column signal fronts for each cycle T is reduced. The timing diagram of FIG. 3 shows the different signals applied to the electrode columns and rows in accordance with a known control process making it possible to reduce coupling phenomena. Signal E represents the signal applied to a row electrode M.sub.i and signals F, G and H represent the signals applied to a column electrode N.sub.j in accordance with the desired display. These signals are square-wave, cyclic signals of cycle T and undergo a polarity reversal for each half-cycle T/2.
The row signal E is of a pulse type and is non-zero for a row time T.sub.L =T/(2.m), m representing the number of rows of electrodes of the display means. To each row m.sub.i is applied a row signal of the same type as row signal E, but delayed by a time t.times.T.sub.L compared with that with 1.ltoreq.t.ltoreq.m-1.
The column signal F is continuous and only has two fronts per cycle T. It is in phase opposition with the pulse of row signal E and consequently permits a white display.
In the same way, column signal G is continuous and has two fronts per cycle. It is in phase with the pulse of row signal E and permits a black display.
However, for a grey display, the corresponding column signal H must retain for each time T.sub.L at least one rising front 15 and on falling front 17 to be successively in phase and in phase opposition with the different pulses of the row signal. Thus, the column signal has 2(m-1) fronts on a cycle T.
The column signal H is delayed by a time .delta. compared with the row signal E. The phase difference .DELTA..phi. between these signals is given by the following relation: EQU .DELTA..phi.=.pi.(.delta./T.sub.L)
Thus, the grey display also leads to coupling phenomena between the electrode rows and the electrode columns to which a column signal H is applied.
Patent application No. 8 022 930 of Oct. 27, 1980 describes a control process making it possible to display several grey levels. In this process, on electrode rows are applied cyclical signals of cycle T and zero mean value. For each cycle T, these signals are divided into n different time fractions, so that the kth time fraction is equal to 2.sup.k-1 .tau., in which k is an integer such that 1.ltoreq.k.ltoreq.n and .tau. is a time interval useful for exciting the material, each time fraction being followed by a dead time u.
To the electrode columns are applied further cyclical signals of cycle T and zero mean value. These signals are also divided into n time fractions, so that the kth time fraction is of the same duration as the kth time fraction of the row signals, said column signals being either in phase, or in phase opposition therewith.
In this control process permitting the display of several grey levels, there are significant coupling phenomena between the rows and columns of electrodes leading to a deterioration of the image displayed.